This invention relates to atmospheric transmission measurements and to means for determining the component of infrared atmospheric attenuation arising from aerosol extinction. Specifically, the invention comprehends an extinctiometer for measuring the infrared extinction coefficient of atmospheric aerosols.
The greatest uncertainty in the application of computer models to the on-site calculation of the transmission of infrared radiation over optical paths of military significance in tropospheric situations is in the wavelength dependent attenuation of optical radiation by atmospheric aerosol particles. The attenuations due to the molecular constitutents of the atmosphere are known in the case of the uniformly mixed gases (N.sub.2, C.sub.2, CO.sub.2, N.sub.2 O, CO, CH.sub.4) or are readily calculable from standard meteorological measurements in the cases of the non-uniformaly mixed molecules (H.sub.2 O, O.sub.3). No suitable instrument appears to be available which will measure atmospheric aerosol extinction coefficients over the environmental range required for military requirements. Measurements from such an instrument, if one existed, would provide invaluable inputs for on-site model predictions of infrared atmospheric transmissions.
There currently exists the need for a method and means for determining the component of infrared atmospheric attenuation arising from aerosol extinction using measurements made with equipment located at a single point on or near a transmission path of interest. The single-point constraint rules out conventional transmissometers as well as bi-static or multi-static laser or searchlight measurements.
For military applications such an instrument is required to be a compact device, operable in outdoor environments from surface levels to 5 km altitude and is to have sufficient dynamic range of sensitivity to cover conditions ranging from fog to 50 km visibility. The preferred technique is a direct measurement of aerosol extinction coefficients although the possibility of separate determinations of aerosol scattering and absorption coefficients is not ruled out. The wavelength region of interest is typically 1-12 micrometers with emphasis on the 1, l the 3-5 and the 8-12 micrometer regions. The instrument must sample ambient air which, in the process of sampling, is unperturbed in terms of temperature, relative humidity, number density of aerosol particles and their size distribution. It must be operable unattended for long periods, without frequent operator intervention for calibration or repair.
It is required that equipment located at one point make measurements from which the component of atmospheric attenuation due to aerosol extinction along a path can be deduced. Solutions to this problem fall into several classes. One class of solutions involves measuring atmospheric transmission. A measurement of atmospheric transmission convolves both the aerosol and molecular components of atmospheric attenuation. The attenuation due only to aerosol extinction is then derivable if the attenuation due to molecular constituents can be estimated or calculated.
Two other classes of solutions are (1) a direct measurement of the optical attenuation parameters of aerosols; i.e. the aerosol extinction coefficient, or separate measurements of the scattering and absorption coefficients, and (2) measurement of the physical parameters of the aerosols with a derivation of the optical extinction.
The simplest method for measurement of atmospheric transmission involves targets of opportunity such as hills, barns, skyscrapers, lighthouses, cathedrals, ships, islands, and the sea horizon. These are the natural visibility targets which have been used for the rather subjective visibility estimates made at airports, and at sea. Instruments exist with which transmission along the path to such a target can be determined by measuring the contrast of the target against the horizon sky background. In general this is a daytime technique and in practice its use has been limited to the visible portion of the spectrum. Such observations are passive in that no optical radiations are projected from the measuring point. Passive observations are sometimes desirable because of the security; i.e., they do not expose the location of the measuring equipments.
A second method for determining the transmission of an atmospheric path involves use of single-ended active equipment. The equipment involves both a projector of radiation along the path and a detector of radiation backscattered by the atmosphere. Usually short laser pulses are used and the time history of backscattered radiation is analyzed to deduce the transmission of the path. Such lidar systems can provide values of path transmission which are based on physical measurements and are calculated using some approximations and assumptions. Paths so investigated can be horizontal, vertical, or slant and the results obtained do not depend on a model of the atmosphere. Lidar systems, which might meet some of the requirements stated above are massive and require frequent, if not constant, attention from an operator. Furthermore they are active systems and not secure.
Measurements of atmospheric aerosol extinction can take two different approaches. In a first approach separate local values of the scattering coefficient and the absorption coefficient can be measured or the local value of the extinction coefficient (which is the sum of the scattering coefficient and the absorption coefficient) at the wavelength or within the wavelength band of interest can be measured. If measurements are made at only one point on the path of interest than an atmospheric model is required to generate values of the extinction coefficient for the other points along the path. Usually such models are based on changes of aerosol characteristics with changes of atmospheric pressure, temperature and relative humidity as well as changes of the aerosol number density, size distribution and complex refractive index and these, in turn, cause changes in values of the extinction coefficient. A second approach, the measurement of the physical parameters of aerosols is similar to the one just outlined except that physical parameters rather than optical parameters of the atmosphere are measured at one end of the path in question. The physical parameters are aerosol size distribution, number density and composition. Rather than composition of the aerosols their complex refractive index is usually given since this is the critical aspect of composition if the aerosol particles are treated as homogeneous spheres. Knowledge of the physical parameters results in somewhat better correlations of changes in the calculated extinction coefficients to changes in the meteorological conditions. The extinction coefficient at any wavelength can be calculated from the data on aerosol size distribution and concentration if only the real and the imaginary indices of refraction are known at each wavelength and for the various sizes of aerosols. Measuring these indices is difficult and it is usual practice to assign index values appropriate to the weather, the season, the history of volcanic action and the region of the world for which the calculations are being made. The changes in size distribution which occur with changes of meteorological parameters (pressure, temperature, relative humidity) can be predicted if the composition of the aerosols is known. The complex index of refraction is also a function of composition. It may be argued that measurement of the physical parameters of the aerosols is more basic than measurement of optical parameters and this is so. However, the real time measurement of aerosol composition which is required to calculate complex refractive index as a function of wavelength is a difficult task. The complex index is very important throughout most of the infrared where values of the single scattering albedo, which is a function of the imaginary index, are significantly less than one. In view of the stringent device requirements given and the various deficiencies attendant to "measurement of contrast", "Lidar" and "point measurements of physical parameters" techniques "point measurement of optical parameters of the atmospheric aerosols" is seen to be the most suitable approach to realizing the extinction measurement requirements.
An extensive literature survey has revealed that the various possible instruments for fulfilling the extinction measurement requirements can be grouped into five main groups as follows:
1. Extinctiometer PA2 2. Transmissometer PA2 3. Nephelometer PA2 4. Spectophone PA2 5. Lidar
The first of these is the subject of the present invention and will hereinafter be described in detail. The literature survey has not found any reference to an equivalent device, indicating that operable extinctiometers of the type considered are presently beyond the state of the art. The extinctiometer is thus unique both in its approach to the measurements problem and in the fact that no known effort has been made to construct a workable instrument based on this concept. Transmissometers exist in a variety of configurations, commonly employing a light source separated from a receiver by the scattering/absorbing medium being measured. The spectral region covered is determined by the light source/filter combination. Long-path transmissometers violate the requirements for compactness, leaving for consideration only the so-called White cell instruments. These employ multiple reflections to achieve a folded path, which approaches the conditions of the long path devices within the confines of a small instrument. The combination of both scattering and absorption, i.e., extinction, is measured. In contrast to this, nephelometers measure only the scattering component of extinction. A number of highly sensitive varieties of this instrument have been developed, including the integrating, polar, and fixed angle nephelometers. Some of these enable separation of the aerosol scattering component by reference to filtered ambient air. Most operate in the visible or near IR spectral regions and make use of incoherent light sources. Spectophones, which measure only the absorption component of extinction, make use of the pressure rise in the absorbing gas due to its being heated by the absorption process. Pressure rise is sensed either with a manometer or microphone, yielding a highly sensitive means of measuring the absorption coefficient. A great variety of spectrophone devices exist, including pulsed and CW types, and these instruments have been operated over a wide spectral region from the visible to the mid IR at 10 .mu.m. Although most spectrophones employ lasers, this is not essential to their operation.
The final instrument type here considered is the lidar, of which there are many varieties. These narrow-beam laser devices measure the backscattered signal from atmospheric scattering and, under proper circumstances, serve to derive the atmospheric transmission for a portion of the laser beam path. In effect, these devices can become single ended transmissometers. Most lidars are short-pulse devices, although one variant is used in CW mode. Some employ Doppler techniques, and some make use of a Raman scatter from specific molecules. Lidars have undergone extensive and highly sophisticated development in the course of the past decade, and show promise for a potentially unique capability for application to the aerosol extinction problem. However, their development at the present time does not satisfy some of the operational requirements, i.e. those related to compactness, field worthiness, and maintainability.
To summarize the above instrument groupings: The extinctiometer and the various transmissometer instruments, including lidars, provide a direct measurement of atmospheric extinction of transmission. The extinctiometer and some transmissometers provide for separating the aerosol extinction from that of the ambient air, whereas in general, lidars do not provide such a separation. Both nephelometers and spectrophones are highly developed, ultra-sensitive, devices for measuring atmospheric scattering (nephelometers) and absorption (spectrophones). For an extinction determination it would be necessary to employ a combination of these two instrument types. Each individually has the advantage of a high state of development and well-demonstrated sensitivity at the levels required for the present application. Neither instrument type requires the use of a laser, although most spectrophone applications have to date employed laser sources. Both nephelometers and spectrophones have been used in a differential mode which separate the effects of aerosols from those of ambient air. Whereas spectrophones have been extensively exploited over the full spectral range required (1-12 microns), most nephelometer applications have been in the visible and near IR regions.
It is seen therefore that the current state-of-the-art does not provide suitable means for obtaining effective accurate atmospheric transmission measurements for the requirements and conditions stated above. Accordingly, there presently exists the need for an extinctiometer that satisfies these requirements. The present invention is directed toward satisfying that need.